Recently, hybrid vehicles, with both an engine and a motor as the driving power source, have attracted attention due to their improvement in fuel efficiency and environmental measures. The hybrid vehicles selectively and appropriately drive one of the engine and the motor or simultaneously drive both the engine and the motor in accordance with the driving conditions. Thus, the hybrid vehicles can achieve higher fuel efficiently and lower gas emission than vehicles that are driven only by an engine.
A hybrid vehicle power transmission device, for example, has proposed a device that performs the following operation when starting an engine by increasing the torque of a traction motor while the vehicle is being driven by the motor. The device increases the rotational speed of the engine by operating a clutch (first clutch) to start the engine. The device selects a second clutch. The second clutch has the maximum transmission torque capacity among clutches of the automatic transmission to form gear steps. The device controls the transmission torque capacity of the second clutch by slip-engaging the second clutch (see, Japanese Unexamined Patent Application Publication No. 2007-131070). Also, a device has been proposed that performs the following operation when starting an engine by using rotation energy of a traction motor while the vehicle is being driven by the motor. The device increases the rotation speed of the motor while slip-controlling a starting clutch (second clutch). Also, it engages a clutch to start the engine when the rotation speed of the motor reaches a predetermined rotation speed (see, Japanese Unexamined Patent Application Publication No. 2000-255285).